Java实现二叉树的示例代码(递归&迭代)

//基础二叉树实现 //使用左右孩子表示法 import java.util.*; import java.util.Deque; public class myBinTree { private static class TreeNode{ char val; TreeNode left; TreeNode right; public TreeNode(char val) { this.val = val; } } public static TreeNode build(){ TreeNode nodeA=new TreeNode('A'); TreeNode nodeB=new TreeNode('B'); TreeNode nodeC=new TreeNode('C'); TreeNode nodeD=new TreeNode('D'); TreeNode nodeE=new TreeNode('E'); TreeNode nodeF=new TreeNode('F'); TreeNode nodeG=new TreeNode('G'); TreeNode nodeH=new TreeNode('H'); nodeA.left=nodeB; nodeA.right=nodeC; nodeB.left=nodeD; nodeB.right=nodeE; nodeE.right=nodeH; nodeC.left=nodeF; nodeC.right=nodeG; return nodeA; } //方法1(递归) //先序遍历: 根左右 public static void preOrder(TreeNode root){ if(root==null){ return; } System.out.print(root.val+" "); preOrder(root.left); preOrder(root.right); } //方法1(递归) //中序遍历 public static void inOrder(TreeNode root){ if(root==null){ return; } inOrder(root.left); System.out.print(root.val+" "); inOrder(root.right); } //方法1(递归) //后序遍历 public static void postOrder(TreeNode root){ if(root==null){ return; } postOrder(root.left); postOrder(root.right); System.out.print(root.val+" "); } //方法2(迭代) //先序遍历 (迭代) public static void preOrderNonRecursion(TreeNode root){ if(root==null){ return ; } Deque stack=new LinkedList<>(); stack.push(root); while (!stack.isEmpty()){ TreeNode cur=stack.pop(); System.out.print(cur.val+" "); if(cur.right!=null){ stack.push(cur.right); } if(cur.left!=null){ stack.push(cur.left); } } } //方法2(迭代) //中序遍历 (迭代) public static void inorderTraversalNonRecursion(TreeNode root) { if(root==null){ return ; } Deque stack=new LinkedList<>(); // 当前走到的节点 TreeNode cur=root; while (!stack.isEmpty() || cur!=null){ // 不管三七二十一，先一路向左走到根儿~ while (cur!=null){ stack.push(cur); cur=cur.left; } // 此时cur为空，说明走到了null，此时栈顶就存放了左树为空的节点 cur=stack.pop(); System.out.print(cur.val+" "); // 继续访问右子树 cur=cur.right; } } //方法2(迭代) //后序遍历 (迭代) public static void postOrderNonRecursion(TreeNode root){ if(root==null){ return; } Deque stack=new LinkedList<>(); TreeNode cur=root; TreeNode prev=null; while (!stack.isEmpty() || cur!=null){ while (cur!=null){ stack.push(cur); cur=cur.left; } cur=stack.pop(); if(cur.right==null || prev==cur.right){ System.out.print(cur.val+" "); prev=cur; cur=null; }else { stack.push(cur); cur=cur.right; } } } //方法1(递归) //传入一颗二叉树的根节点，就能统计出当前二叉树中一共有多少个节点，返回节点数 //此时的访问就不再是输出节点值，而是计数器 + 1操作 public static int getNodes(TreeNode root){ if(root==null){ return 0; } return 1+getNodes(root.left)+getNodes(root.right); } //方法2(迭代) //使用层序遍历来统计当前树中的节点个数 public static int getNodesNoRecursion(TreeNode root){ if(root==null){ return 0; } int size=0; Deque queue=new LinkedList<>(); queue.offer(root); while (!queue.isEmpty()) { TreeNode cur = queue.poll(); size++; if (cur.left != null) { queue.offer(cur.left); } if (cur.right != null) { queue.offer(cur.right); } } return size; } //方法1(递归) //传入一颗二叉树的根节点，就能统计出当前二叉树的叶子结点个数 public static int getLeafNodes(TreeNode root){ if(root==null){ return 0; } if(root.left==null && root.right==null){ return 1; } return getLeafNodes(root.left)+getLeafNodes(root.right); } //方法2(迭代) //使用层序遍历来统计叶子结点的个数 public static int getLeafNodesNoRecursion(TreeNode root){ if(root==null){ return 0; } int size=0; Deque queue=new LinkedList<>(); queue.offer(root); while (!queue.isEmpty()){ TreeNode cur=queue.poll(); if(cur.left==null && cur.right==null){ size++; } if(cur.left!=null){ queue.offer(cur.left); } if(cur.right!=null){ queue.offer(cur.right); } } return size; } //层序遍历 public static void levelOrder(TreeNode root) { if(root==null){ return ; } // 借助队列来实现遍历过程 Deque queue =new LinkedList<>(); queue.offer(root); while (!queue.isEmpty()){ int size=queue.size(); for (int i = 0; i